An automatic repeat request (ARQ) is a mechanism by which a receiving end of a wireless connection can request retransmission of a media access control (MAC) protocol data unit (PDU), generally as a result of having received the PDU with errors. The ARQ mechanism is a part of the IEEE 802.16 MAC layer and can be enabled on a per-connection basis.
Under low signal-to-noise ratio (SNR) conditions in wireless networks, various techniques can be used to keep packet error rate (PER) under control. Some of these techniques include a transmitter using better modulation, coding, and repetition. For example when a low SNR is detected, a transmitter may use quadrature phase-shift keying (QPSK) modulation with a forward error correction (FEC) rate of one-half and repetition rate of two.
Another technique to control PER under low SNR conditions includes hybrid ARQ (HARQ) burst chase combining. For example, a receiver may use multiple retransmissions of the same burst to converge on a maximum likelihood decoding of FEC blocks. This technique is considered to be part of the physical layer. After FEC decoding, cyclic redundancy check (CRC) data can be checked to decide on the fate of the burst.
Still another technique to control PER under low SNR conditions includes using ARQ at the MAC layer to retransmit any packets in error. If HARQ is used, CRC on the HARQ burst would not be used in this case, since the ARQ blocks have CRC. In general, a burst, HARQ or not, will have multiple ARQ blocks.
Finally, to control PER under low SNR conditions, the transmitter can vary its power based on SNR feedback from the receiver. If the above techniques fail to provide an acceptable PER, a transmitter typically sends a resynchronization message to the receiver.
In spite of all the above techniques, there can still be low coverage areas in wireless networks where PER is unacceptably high.